In general when light strikes a surface, some of it may be reflected, some absorbed, some scattered, and the rest transmitted. Reflection can be diffuse, such as light reflecting off a white wall, or specular, as in light reflecting off a mirror. An opaque substance transmits almost no light, and therefore reflects, scatters, or absorbs all of it. Both mirrors and carbon black are opaque. Opacity depends on the frequency of the light being considered. “Blackout” or light blocking materials typically refer to coated layers in articles that are substantially impermeable to light such as visible or UV radiation. Thus, when a blackout material such as a blackout curtain is hung over a window, it generally blocks substantially all external light from entering the room through that window. Blackout materials are suitable as curtains for domestic use, for institutional use in hospitals and nursing homes, as well as for use in commercial establishments such as hotels, movie theaters, and aircraft windows where the option of excluding light from a room can be desirable.
The human eye has about 12 or 13 orders of magnitude dynamic range in detecting light intensity and it can easily detect conditions ranging from noon sun on new snow at high elevations with no clouds or haze, to moonless night with some haze. Normal sunlight is about 8 orders of magnitude brighter than starlight but only about 3 orders brighter than a typical living room and 2 orders brighter than a typical office. Thus for example, it is desirable for a blackout material to reduce transmitted sunlight into a living room by at least 3 orders of magnitude for lighted observation such as looking at screens of televisions, computers, or mobile telephones or other devices and by at least 5 orders of magnitude for activities requiring further darkening such as sleeping. The presence of external sunlight reflectors such as snow or sand requires an even greater extent of light blocking.
The measure of the extent to which a substance transmits light or other electromagnetic radiation is given by the transmission density (Dt), also known as optical density, and is equal to the logarithm to base ten of the reciprocal of the transmittance. The transmission of electromagnetic radiation through an absorbing medium in a coated layer, with an absorption coefficient that characterizes how readily a material or medium can be penetrated by a beam of light, follows Beer's law (the linear relationship between log of the absorbance and concentration of an absorber of electromagnetic radiation). The coating exhibits an exponential decay in the intensity of transmitted light with increased thickness of the coated layer. The characteristic penetration depth of electromagnetic radiation into the coated layer (the reciprocal of the absorption coefficient) is a measure of how deep the electromagnetic radiation can penetrate the coated layer before it is stopped, and is defined as the depth at which the intensity of the radiation inside the coated layer falls to 1/e (about 37%) of its original value just below its surface. Depending on the nature of the coated layer, the electromagnetic radiation might travel very far into the coated layer or it might be blocked quickly. When the electromagnetic radiation passes through media that have both scattering and absorbing properties, the radiation can be further weakened or attenuated.
Light blocking articles such as the blackout curtains can be comprised of a fabric coated with polymeric latex foams. There is a desire for these curtains to have a light color (hue) facing the environment when an activity needs illumination so as to minimize the amount of artificial lighting needed to perform the activity. Additionally, having a light colored back side is desirable. An example is when the function of the blackout material is to separate two areas of activity where one or both areas can be artificially lit at the same time. More often, the function of a blackout curtain is to prevent sunlight from entering a room through a building window. It can also be desirable for the color (hue) of the back side to match the external decor of the building.
Light colored blackout curtains can be made by coating a fabric with light colored foams containing light scattering pigments such as titanium dioxide or clays. Because light is scattered more forward than backward from any light scattering pigment, very thick foam coatings are required to create blackout curtains through which the sun is not visible in a darkened room. These light scattering pigments are heavy in weight and require a special fabric to cover a window. One method that is used to reduce the weight of such blackout materials is to sandwich a light-absorbing carbon black layer between two light scattering layers.
In other applications of such materials, an opacifying layer can be used to hide an undesirable colored material beneath it. In these instances, where reflected light is observed after it enters an opacifying layer, the light travels though the opacifying layer twice as it is reflected back from a substrate. This effect reduces the required light blocking optical density by 50% of what is required for the situation when transmitted radiation is observed in blackout materials.
Vesiculated polymer particles have been used as replacements for light scattering pigments such as titanium dioxide. The large difference in refractive indexes between the entrapped air and the polymer walls of the particles causes light scattering and contributes to the hiding power and white appearance of the resulting opacifying coating. With this optical phenomenon, opacity and whiteness arises from the interaction of light with a multiplicity of interfaces and microvoids. U.S. Pat. No. 7,572,846 (Engelbrecht et al.) describes a method for the manufacture of vesiculated polymer particles that are suitable for the replacement of titanium dioxide pigments and extenders, and that are said to have opacity, whiteness, scrub resistance, and water resistance.
U.S. Pat. No. 4,677,016 (Ferziger et al.) describes a flame retardant, drapeable, and substantially light impermeable fabric that is considered suitable for use as a curtain, window shade, or other window covering and comprises foam coating compositions in which one of the foam coated layers is opaque and is comprised of a cured layer of a polymer latex foam.
U.S. Pat. No. 4,457,980 (Daniels et al.) discloses highly opaque printed areas on uncolored or pre-colored fabrics with the use of an aqueous opaque printing paste comprising a dispersion of an opacifying pigment and an aqueous curable latex polymer binder.
U.S. Pat. No. 5,576,054 (Brown) describes silicone rubber compositions and a method of opacifying a spandrel glass surface, to stop light transmission by applying to the surface a coating composition comprising an ultraviolet light resistant organopolysiloxane and an opacifying agent that is a mixture of carbon black and titanium dioxide present in the amount of 1 to 25% by weight and in a ratio of between 1:10 and 1:100 by weight of carbon black to titanium dioxide, in an amount sufficient to provide sufficient surface opacity to light transmission, and curing the coating composition on the surface.
U.S. Pat. No. 8,435,340 (Wheeler et al.) describes an aqueous coating composition having a pigment volume content (PVC) of 78% to 88% comprising, based on the total volume solids of the dry coating, opacifying pigment particles comprising: from 3 to 10% titanium dioxide, from 0 to 20% of hollow polymeric particles; non-opacifying extender particles, polymer binder particles of calculated Tg of from 25° C. to 70° C., a dispersant, and a fugitive coalescing solvent.
U.S. Pat. No. 7,754,409 (Nair et al.), U.S. Pat. No. 7,887,984 (Nair et al.), U.S. Pat. No. 8,252,414 (Putnam et al.), and U.S. Pat. No. 8,329,783 (Nair et al.) describe porous polymer particles that are made by a multiple emulsion process, wherein one phase of the process provides formation of individual porous particles comprising a continuous polymer phase and internal pores, and such individual porous particles are dispersed in an external aqueous phase. The described Limited Coalescence process is used to control the particle size and distribution while a hydrocolloid is incorporated to stabilize the inner emulsion that provides the pores in the porous particles.
When an electromagnetic radiation blocking coating has, as it often does, a strongly light absorbing material such as carbon black between two reflective layers, it has at least two distinct problems. First, such materials require three separate coating operations that reduce manufacturing productivity and increase unit costs. Secondly, the light absorbing layer can be exposed to the environment by stitching failure or surface damage of the white reflective coatings and the damaged area will be highly visible against the lightly colored reflective surface. Additionally, the stitching in the materials can generate fugitive material from the light absorbing layer that can be spread over a larger area thereby increasing the area of objectionable shading of the light colored surface. Inorganic pigments typically used for high opacity coatings are high specific gravity pigments that can undesirably settle in the coating solvents. Thus, they are difficult to keep dispersed in the coating formulations and contribute a significant amount of weight to the final coating.
There is therefore a need to replace high specific gravity inorganic pigments such as titanium dioxide, additional extenders, and other high weight additives to provide a radiation-blocking material that is suitable as a blackout material but also light-weight and possesses the supple feel that is characteristic of textiles. There is also a need for radiation-blocking materials that can be readily washed and sewn without carbon black showing through or being exposed to the environment. There is a further need to keep the article thickness and the number of manufacturing operations (for example, coating passes) to a minimum. It is also desirable to have a light colored article with an outer coating that is easily tinted or shaded with additional colorants to meet the user and environmental needs. For example, it is also desirable to provide a means for the color of the radiation-blocking article to match that of external decor.